The present invention is directed to a shape-selective xylene disproportionation process over a surface modified molecular sieve catalyst to selectively form pseudocumene.
The term "shape-selective catalysis" describes the catalytic selectivities found in molecular sieves such as zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., by N.Y. Chen, W. E. Garwood and F. G. Dwyer, Shape Selective Catalysis in Industrial Applications, 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, alkylation or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within zeolite pores or cages. Another type of selectivity results from configurational restraints on diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in the dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in selective toluene disproportionation to para-xylene as disclosed in U.S. Pat. No. 5,659,098 to Beck et al. Another example of shape selective catalysis, ethylbenzene disproportionation, is disclosed in U.S. Pat. No. 5,406,015 to Beck et al.
Xylenes and higher aromatics such as pseudocumene and mesitylene are desired commercial products having various uses as disclosed in N. E. Ockerbloom's article, "Xylenes and Higher Aromatics" in Hydrocarbon Processing, April 1972, pg. 114-118. This article discloses that these products can be obtained through fractional distillation or extraction from C.sub.9 aromatic fractions obtained from naphtha cracking or reformate. Pseudocumene may be oxidized to form trimellitic acid which is useful in the manufacture of synthetic fibers and plastics. It may also be desirable to manufacture the acid anhydride form. Thus, there have been efforts to devise processes resulting in a high purity pseudocumene product. Fractionation of an extracted, heavy catalytic reformate containing about 40% pseudocumene to obtain purified pseudocumene requires large fractionation towers to perform the separation. Using a crystalline zeolite catalyst on an extracted reformate cut to increase pseudocumene content has been proposed in U.S. Pat. No. 5,004,854 to Yan.
Synthesis of pseudocumene by methylation of benzene or methyl-substituted benzenes is disclosed as producing a higher than equilibrium concentration of pseudocumene in U.S. Pat. No. 4,891,467 to Sikkenga. However, the requirement of a methanol feedstock limits the practical application of such a process. U.S. Pat. No. 3,784,621 to Suggitt discloses a process whereby toluene is contacted with a disproportionation catalyst, a C.sub.8 aromatic fraction is then separated, and the C.sub.8 aromatic fraction is then contacted with a disproportionation catalyst to form a C.sub.9 aromatic fraction. The C.sub.9 aromatic fraction may contain pseudocumene and/or mesitylene. Disproportionation of ortho-xylene to produce primarily mesitylene is disclosed by Suggitt et al in U.S. Pat. No. 3,915,895. U.S. Pat. No. 4,038,335 to Minachev et al discloses a catalytic xylene disproportionation process yielding on the order of 68% to 69% pseudocumene, relative to all trimethylbenzene isomers.
It would be desirable to produce a high purity pseudocumene product via a xylene disproportionation process amenable to a wide variety of xylene feeds including single isomer feeds, equilibrium mixtures and para depleted feeds without the requirement of special feedstocks and/or operating conditions, and without the need for further purification to increase the pseudocumene content of the product.